1. Field of the Invention
The present invention relates to a satellite communications system and its handover processing method, and in particular, to a satellite communications system using a group of orbital satellites according to a satellite fixed cell scheme, as well as a handover processing method therefor.
2. Description of the Prior Art
In the current satellite communications system using a group of orbital satellites, a plurality of satellites are arranged around the earth so as to revolve around it along certain orbits. One of known systems that are adapted for such a satellite communications system uses a group of low-orbital satellites that are principally non-stationary orbital satellites such as iridium satellites or global stars which operate on low earth orbits (LEO).
For example, iridium satellites revolve along six orbits that are substantially orthogonal with the equator. Eleven satellites are located on a single orbit so as to construct a cellular telephone network in which a total of 66 satellites cover every area of the earth. A variety of control centers (hereafter referred to as xe2x80x9cground stationsxe2x80x9d) on the earth monitor the satellite network, transmit instructions for attitude control, and inform each satellite of necessary information. An iridium system satellite has a switching function and an intersatellite communication function. If a user makes a telephone call from an iridium terminal to a public telephone network, a radio wave originated from the iridium terminal is first received by an overhead satellite and then passes through adjacent satellites to reach a gateway connected to a destined telephone network. The call further passes through the telephone network connected to the gateway before connecting to the destination.
In the case where a user makes a call from an iridium terminal to a subscriber telephone connected to a public telephone network, a call setup request is first sent to the nearest gateway. This is to authenticate this iridium terminal. Upon identifying the iridium terminal, the gateway then identifies a gateway corresponding to the destination, and delivers the setup request to this gateway via a satellite circuit. The gateway that has received the connection request identifies the destination and makes a call to a corresponding subscriber telephone network to make a connection. When the destination receives the call, it connects to a communication path joining the satellites together and then starts communication. If the iridium terminal is connected to a telephone connected to a public telephone network, the call setup processing involves two gateways.
In the case of communication between iridium terminals, the satellites also provide a switching function and a communication function. Switching processing, however, is not carried out only by the satellites but connections are always made through a circuit connection procedure via gateways.
The orbits of iridium satellites are as described above. Each of the satellites arranged on these orbits covers the overall global surface using beams impinging on the surface of the earth. A plurality of such communication beams emitted from a satellite cooperate with one another to form a coverage area of the global surface. Since the coverage area is formed by non-stationary orbital satellites, it moves relative to the global surface over time. When the iridium terminal is located in the area of a certain overhead satellite, it can connect to another terminal in the same area or to a public telephone network via a gateway in the same area. Alternatively, when the iridium terminal is to be connected to another iridium terminal or a public telephone network that belongs to a different area, a call is set up via an intersatellite communication link established with an adjacent satellite.
The coverage area formed by a plurality of beams from a satellite in corporation is formed of subsections each formed by a corresponding one of the plural beams. By dividing the coverage area into the subsections, the number of communication channels available in the system can be increased to augment the entire capacity (that is, the number of mobile stations that can be supported by the system).
Likewise, each subsection formed by a plurality of cover beams emitted by satellites corresponds to one dynamic geographical area (cell) that moves over the ground surface as the satellite obits the earth. A coverage beam emitted to each cell is assigned a particular frequency as a communication carrier, and each carrier frequency supports a plurality of channels, which each channel assigned to a specific terminal located within the cell. A terminal can communicate with a satellite at an assigned frequency on an assigned communication channel as long as this terminal is located within the associated cell.
When, however, the terminal leaves an old beam/cell and enters a new beam/cell, this terminal must be assigned a new communication channel. In addition, this terminal must adjust its communication frequency to the frequency associated with the new cell. Otherwise, the current communication channel (an ongoing call) will be dropped (that is, disconnected). Once the terminal has entered the new beam/cell, it uses the new channel for all communications with the satellite.
In addition, of course, such switching of communication channels is required not only within the same satellite but also between satellites. In a satellite communications system principally using a group of low-orbital satellites as described above, a terminal may shift from a satellite by which it is being serviced to a service area of a next satellite, and in this case, it must also switch to a new communication channel.
The process of switching channels carried out in this aspect is called xe2x80x9chandover.xe2x80x9d Handovers are necessary when a mobile station moves from one beam or cell to another beam or cell, and in order to carry out handover, the mobile station must switch to a new predetermined transmission or reception frequency.
In the handover process carried out in a conventional mobile communication system, when a terminal determines the necessity of handover, for example, depending on the intensity of electric field received from a satellite, it sends a handover request signal to a gateway belonging to a service area for the terminal or directly to the satellite. Then, the gateway or satellite that has received this signal transmits and receives information to and from the satellite or gateway, respectively, in order to switch to a new channel to the terminal.
To achieve a smooth handover process, timings when handover becomes necessary are preferably known beforehand. For example, as an example of a conventional technique relating to the handover process, a gateway makes analysis based on a report generated by a terminal as appropriate, and if a power level contained in the report has a predetermined threshold or lower, the gateway determines that the terminal is leaving the current coverage area and recognizes that the handover process will be required in the near future. In an alternative known technique, the gateway continuously monitors the position of the terminal and the position of an associated coverage beam to determine a timing when handover becomes necessary. The gateway, which has used the above method to recognize the timing when handover is to be carried out, transmits signals with appropriate timings starting with the handover request signal to establish a new channel between a corresponding satellite and the terminal.
Handover in the conventional intersatellite communications system will be described with reference to the sequence diagram shown in FIGS. 1 and 2.
With reference to FIG. 1, handover between a first satellite and a second satellite following the first satellite on the same orbit will be described. A ground station controls all the satellites such that a controller of each satellite is caused to register via its receiver a physical location immediately below the navigating satellite (the east longitude, the north latitude, or the like), a schedule indicating the corresponding point of time, physical locations immediately below satellites preceding or following the first satellite on its orbit (the east longitude, the north latitude, or the like), and a schedule indicating the corresponding point of time (B1, B2).
When the terminal determines that handover is necessary, for example, depending on the intensity of electric field received from a satellite, it sends a handover request signal to a gateway belonging to a service area for the terminal or directly to the first satellite (B3). Upon receiving the handover request signal, the first satellite starts handover processing via an intersatellite communication link (B4) and transmits a downlink message for the terminal to a second satellite (B5).
The first satellite transmits the last communication data via a communication link established with the terminal (B6), and the second satellite transmits the first communication data (B7). Once a new up-link has been established between the terminal and the second satellite, the terminal transmits the last communication data to the first satellite (B8) and then transmits the first communication data to the second satellite (B9).
With reference to FIG. 2, handovers will be described which are executed in response to a handover request signal received from the terminal in the order: a third satellite, a first satellite, and a second satellite, which are arranged on the same orbit. A ground station controls all the satellites such that a controller of each satellite is caused to register via its receiver a physical location immediately below the navigating satellite (the east longitude, the north latitude, or the like), a schedule indicating the corresponding point of time, physical locations immediately below satellites preceding or following the first satellite on its orbit (the east longitude, the north latitude, or the like), and a schedule indicating the corresponding point of time (B11, B12, B13).
When a terminal covered by the third satellite determines the necessity of handover, for example, depending on the intensity of electric field received from the third satellite, it sends a handover request signal to a gateway belonging to a service area for the terminal or directly to the third satellite (B14). Upon receiving the handover request signal, the third satellite starts the handover procedure for the first satellite via an intersatellite communication link (B15).
When a terminal covered by the first satellite determines the necessity of handover, for example, depending on the intensity of electric field received from the first satellite, it sends a handover request signal to a gateway belonging to a service area for the terminal or directly to the first satellite (B16). Upon receiving the handover request signal, the first satellite starts handover for the second satellite via an intersatellite communication link (B17), and the first satellite executes transmissions to the second satellite (B18). The first satellite transmits the last communication data via the established communication link (B19), while the second satellite transmits the first communication data via a link newly established with the terminal (B20).
In the handover process of the above satellite communications system, a large amount of information is communicated, so that a large amount of time is required to complete this process. Consequently, the resulting adverse effects on communication cannot be avoided.
Further, in the handover process which is started depending on, for instance, the intensity of electric field received from a satellite, it is necessary to include information indicating that the handover is started in the notice to be sent to a gateway or a satellite, resulting in a large amount of data to be transmitted.
Furthermore, an incidental factor may cause the handover process to be started, resulting in unexpected and unavoidable communication failure.
There has been disclosed a satellite-based telecommunications system, which performs scheduled handovers, enabling faster handover process and minimizing interrupts due to the handover process, in Japanese Patent Application Unexamined Publication No. 10-108267.
An object of the present invention is to provide satellite communications system and handover processing method that can achieve the stable and reliable handover process.
According to the present invention, schedule information is previously registered in each of the satellites. The schedule information of each satellite includes a radius of a coverage area thereof, a point of time provided thereto, and physical locations of a center point of the coverage area thereof according to the point of time. After determining a physical location of a first station which is located in a first coverage area of a first satellite, a handover start time is calculated based on the schedule information and the physical location of the first station and a control signal regarding the handover is transmitted to an adjacent satellite. Since the handover start time is known in advance at both the first satellite and the adjacent satellite, the handover can be performed and completed between them without sending handover request signal.
According to a first aspect of the present invention, a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link, each of the satellites comprising: a ground communication section for communicating with stations located in the coverage areas on the earth; an intersatellite communication section for communicating with other satellites through intersatellite communications links; and a controller controlling the ground and intersatellite communication sections to perform a handover process, wherein in a first satellite forming a coverage area in which the station is located, a first controller calculates a handover start time at which the handover process should be started for the station, and controls the intersatellite communication section so as to transmit a control signal having a handover completion time added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time is calculated from the handover start time, and in a second satellite receiving the control signal having the handover completion time added thereto from the first satellite, a second controller performs the handover process by establishing a new communication link to the station before the handover completion time.
According to a second aspect of the present invention, in a first satellite forming a coverage area in which the station is located, a first controller calculates a handover start time at which the handover process should be started for the station, and controls a first intersatellite communication section so as to transmit a control signal having a handover completion time added thereto and communication data received from the station at the handover start time and having next handover-related data added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time is calculated from the handover start time, and in a second satellite receiving from the first satellite the control signal having the handover completion time added thereto and the communication data having the next handover-related data added thereto, a second controller performs the handover process by establishing a new communication link to the station before the handover completion time, compares communication data received from the station using the new communication link with the next handover-related data to determine whether the communication data and the next handover-related data are in sequence, and controls second ground and intersatellite communication sections such that a sequence of communication data is received from the station and is transmitted toward an opposite-side station communicating with the station.
According to a third aspect of the present invention, a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite, each of the satellites comprising: a ground communication section for communicating with stations on the earth; an intersatellite communication section for communicating with other satellites through intersatellite communications links; a storage device for storing data received from other satellites through the intersatellite communications links; a selector for selecting one of data received by the ground communication section and data stored in the storage device; and a controller controlling the ground and intersatellite communication sections and the selector to perform a handover process, wherein in a first satellite forming a coverage area in which the station is located, a first controller calculates a handover start time Txe2x88x92t at which the handover process should be started for the station, wherein T is a handover completion time and t is a time required for the handover process, and controls a first intersatellite communication section so as to transmit a control signal having the handover completion time T added thereto and communication data received from the station at the handover start time Txe2x88x92t and having next handover-related data added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time T is calculated from the handover start time Txe2x88x92t, and in a second satellite receiving from the first satellite the control signal having the handover completion time T added thereto and the communication data having the next handover-related data added thereto, a second controller performs the handover process by establishing a new communication link to the station before a lapse of the handover completion time T, inputing data from the second ground communication section using the new communication link, inputting data from a second storage device when the selector is switched by the second intersatellite communication section at the handover completion time T, comparing communication data received from the station using the new communication link with the next handover-related data to determine whether the communication data and the next handover-related data are in sequence, and controls second ground and intersatellite communication sections such that a sequence of communication data is received from the station and is transmitted toward an opposite-side station communicating with the station.
According to a fourth aspect of the present invention,a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link, each of the satellites comprising: a ground communication section for communicating with stations located in the coverage areas on the earth; an intersatellite communication section for communicating with other satellites through intersatellite communications links; and a controller controlling the ground and intersatellite communication sections to perform a handover process, wherein in a first satellite forming a coverage area in which a station is located, a first controller calculates a handover start time at which the handover process should be started for the station, and controls the intersatellite communication section so as to transmit a control signal having a handover completion time added thereto to a second satellite that is to be a handover destination satellite adjacent to the first satellite using a first intersatellite communication link and a third satellite which is communicating with the first satellite using a second intersatellite communication link, wherein the handover completion time is calculated from the handover start time, and in the second satellite receiving the control signal having the handover completion time added thereto from the first satellite, a second controller performs the handover process by establishing a new communication link to the station before the handover completion time, receiving communication data from the station using the new communication link, and transmitting the received communication data to the third satellite using a third intersatellite communication link.
According to a fifth aspect of the present invention, a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link, each of the satellites comprising: a ground communication section for communicating with stations located in the coverage areas on the earth; an intersatellite communication section for communicating with other satellites through intersatellite communications links; and a controller controlling the ground and intersatellite communication sections to perform a handover process, wherein in a first satellite forming a coverage area in which the station is located, a first controller calculates a handover start time Txe2x88x92t at which the handover process should be started for the station, wherein T is a handover completion time and t is a time required for the handover process, controls a first intersatellite communication section so as to transmit a control signal having the handover completion time T added thereto to a second satellite that is to be a handover destination satellite adjacent to the first satellite using a first intersatellite communication link and a third satellite which is communicating with the first satellite using a second intersatellite communication link, and controls the first intersatellite communication section so as to transmit first communication data having next handover-related data added thereto to the third satellite at the handover star time Txe2x88x92t, wherein the handover completion time is calculated from the handover start time, and in the second satellite receiving the control signal having the handover completion time T added thereto from the first satellite, a second controller performs the handover process by establishing a new communication link to the station before the handover start time Txe2x88x92t, and transmitting second communication data to the third satellite using a third intersatellite communication link, wherein the second communication data is received from the station using the new communication link, in the third satellite receiving the first communication data from the first satellite and the second communication data from the second satellite, a third controller compares the first communication data having the next handover-related data added thereto with the second communication data to determine whether these received communication data are in sequence, and controls third ground and intersatellite communication sections such that a sequence of communication data received from the station is transmitted toward an opposite-side station communicating with the station.
According to the present invention, a handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link, each of the satellites including: a ground communication section for communicating with stations located in the coverage areas on the earth; an intersatellite communication section for communicating with other satellites through intersatellite communications links; and a controller controlling the ground and intersatellite communication sections to perform a handover process, the method includes the steps of: at a first satellite forming a coverage area in which the station is located, a) calculating a handover start time at which the handover process should be started for the station; b) transmitting a control signal having a handover completion time added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time is calculated from the handover start time; at a second satellite receiving the control signal having the handover completion time added thereto from the first satellite, c) performing the handover process by establishing a new communication link to the station before the handover completion time.
According to another aspect of the present invention, a handover processing method includes the steps of: at a first satellite forming a coverage area in which the station is located, a) calculating a handover start time at which the handover process should be started for the station; b) transmitting a control signal having a handover completion time added thereto and communication data received from the station at the handover start time and having next handover-related data added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time is calculated from the handover start time; at a second satellite receiving from the first satellite the control signal having the handover completion time added thereto and the communication data having the next handover-related data added thereto, c) establishing a new communication link to the station before the handover completion time; d) comparing communication data received from the station using the new communication link with the next handover-related data to determine whether the communication data and the next handover-related data are in sequence; e) receiving a sequence of communication data from the station; and f) transmitting the sequence of communication data toward an opposite-side station communicating with the station.
According to still another aspect of the present invention, a handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite, each of the satellites comprising: a ground communication section for communicating with stations on the earth; an intersatellite communication section for communicating with other satellites through intersatellite communications links; a storage device for storing data received from other satellites through the intersatellite communications links; a selector for selecting one of data received by the ground communication section and data stored in the storage device; and a controller controlling the ground and intersatellite communication sections and the selector to perform a handover process, the method includes the steps of: at a first satellite forming a coverage area in which the station is located, a) calculating a handover start time Txe2x88x92t at which the handover process should be started for the station, wherein T is a handover completion time and t is a time required for the handover process; b) transmitting a control signal having the handover completion time T added thereto and communication data received from the station at the handover start time Txe2x88x92t and having next handover-related data added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time T is calculated from the handover start time Txe2x88x92t; at a second satellite receiving from the first satellite the control signal having the handover completion time T added thereto and the communication data having the next handover-related data added thereto, c) establishing a new communication link to the station before a lapse of the handover completion time T, d) inputing data from the second ground communication section using the new communication link; e) inputting data from a second storage device when the selector is switched by the second intersatellite communication section at the handover completion time T; f) comparing communication data received from the station using the new communication link with the next handover-related data to determine whether the communication data and the next handover-related data are in sequence; g) receiving a sequence of communication data from the station; and h) transmitting the sequence of communication data toward an opposite-side station communicating with the station.
According to further another aspect of the present invention, a handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link, each of the satellites comprising: a ground communication section for communicating with stations located in the coverage areas on the earth; an intersatellite communication section for communicating with other satellites through intersatellite communications links; and a controller controlling the ground and intersatellite communication sections to perform a handover process, the method includes the steps of: at a first satellite forming a coverage area in which a station is located, a) calculating a handover start time at which the handover process should be started for the station; b) transmitting a control signal having a handover completion time added thereto to a second satellite that is to be a handover destination satellite adjacent to the first satellite using a first intersatellite communication link and a third satellite which is communicating with the first satellite using a second intersatellite communication link, wherein the handover completion time is calculated from the handover start time; at the second satellite receiving the control signal having the handover completion time added thereto from the first satellite, c) performing the handover process by establishing a new communication link to the station before the handover completion time; d) receiving communication data from the station using the new communication link; and e) transmitting the received communication data to the third satellite using a third intersatellite communication link.
According to still further another aspect of the present invention, a handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link, each of the satellites comprising: a ground communication section for communicating with stations located in the coverage areas on the earth; an intersatellite communication section for communicating with other satellites through intersatellite communications links; and a controller controlling the ground and intersatellite communication sections to perform a handover process, the method includes the steps of: at a first satellite forming a coverage area in which the station is located, a) calculating a handover start time Txe2x88x92t at which the handover process should be started for the station, wherein T is a handover completion time and t is a time required for the handover process; b) transmitting a control signal having the handover completion time T added thereto to a second satellite that is to be a handover destination satellite adjacent to the first satellite using a first intersatellite communication link and a third satellite which is communicating with the first satellite using a second intersatellite communication link; c) transmitting first communication data having next handover-related data added thereto to the third satellite at the handover star time Txe2x88x92t, wherein the handover completion time is calculated from the handover start time; at the second satellite receiving the control signal having the handover completion time T added thereto from the first satellite, d) establishing a new communication link to the station before the handover start time Txe2x88x92t; e) transmitting second communication data to the third satellite using a third intersatellite communication link, wherein the second communication data is received from the station using the new communication link; at the third satellite receiving the first communication data from the first satellite and the second communication data from the second satellite, f) comparing the first communication data having the next handover-related data added thereto with the second communication data to determine whether these received communication data are in sequence; and g) transmitting a sequence of communication data received from the station toward an opposite-side station communicating with the station.